Tacoma fault - News Tribune 3 Nov 2003

The News Tribune - Tacoma, WA Monday, November 3, 2003

Tacoma Fault poses a threat
STEVE MAYNARD; The News Tribune

Researchers Sunday unveiled their first firm evidence that a Tacoma fault line exists and that it's larger than they suspected.

Now, the challenge is to home in on where the Tacoma Fault runs. In the winter, researchers will study the White River Fault near Enumclaw and whether it connects with the Tacoma Fault, Brian Sherrod of the U.S. Geological Survey told an international conference of geoscientists in Seattle.

The Tacoma Fault "adds yet another reminder that we live in an area that's prone to big earthquakes," said Sherrod, who is also affiliated with the University of Washington. "It adds another piece of the tectonic puzzle."

Does finding the Tacoma Fault mean the South Sound is even more prone than previously thought to a major earthquake? Sherrod dodged that question.

"I don't want to go there," he said during an interview. "We can't predict when earthquakes are going to occur."

The best scientists can do is describe where the fault exists and how it moves. "The work is just beginning on the Tacoma Fault," Sherrod said.

Here's what researchers think they know so far: The fault zone stretches 50 kilometers - about 31 miles - and travels from the southeastern tip of Hood Canal, across the southern tip of Vashon Island north of the Tacoma Narrows, and extends either through Commencement Bay, Federal Way, or somewhere in between.

Sherrod and John Crouch, who recently graduated from the UW, discussed their findings at one of many breakout sessions at the annual meeting of the Geological Society of America. An estimated 7,000 geoscientists are expected to attend the gathering, which started Sunday and continues through Wednesday at the Washington State Convention & Trade Center.

While the annual meeting is for geoscientists, Sherrod will give a public talk on "Faults beneath our feet: Earthquakes in the Puget Sound region" from 7:30-9:30 p.m. Tuesday at the Sheraton Hotel, 1400 Sixth Ave., Seattle.

Crouch displayed high-tech images showing ruptures from an earthquake 20 to 40 meters below sea bottom in Carr Inlet near Gig Harbor. This shallow faulting is evidence of a major earthquake thought to have occurred around 1,100 years ago, said Crouch, a 27-year-old Tacoman who did his research as an undergraduate oceanography student at the UW. The Tacoma Fault has the potential to move with another major fault line to the north, the Seattle Fault.

Crouch said the Tacoma Fault is "a real danger because it's so close to the surface," and able to produce major rupturing. "This is the scariest of all the earthquakes that could happen in the region," Crouch said.

Crouch and Sherrod addressed about 100 people in one of numerous sessions at the geoscientists' meeting. Sherrod showed high-tech aerial images of scarps, areas where uplifting occurred due to faults, as a well as a picture of a rising coastline in Case Inlet near Allyn - all evidence of uplifting in the Earth and faulting.

Researchers believe the Tacoma Fault may have caused a shallow earthquake of 7 to 7.5 magnitude 1,100 years ago, Sherrod said. They believe that earthquake caused areas of uplift along the Tacoma Fault of 4 meters, or about 12 feet.

If the Tacoma Fault were to rupture along its entire known length of 50 kilometers, it could produce an earthquake of 6.5 to 7.5 magnitude, equivalent to the Northridge, Calif. quake in 1994, Sherrod said.

His research on the Tacoma Fault goes back to 1994. Last year, designers of the new Tacoma Narrows bridge urged Sherrod and other researchers to investigate the scarp, or uplifting of the ground surface, that they found in high-tech images.

"We have suspected that there's been a Tacoma Fault for a long time," said Sherrod, who lives in Federal Way. Until now, "we lacked the conclusive evidence where the fault was."

All this made fascinating listening for Courtney Johnson, 21, a geoscience major at Pacific Lutheran University. She is considering doing her senior project on the Tacoma Fault.

"I think it's interesting that it's been suddenly discovered," Johnson said.

As to whether the Tacoma Fault increases the risk of a major earthquake in the South Sound, she shrugged. That risk, she said, is "always there."

Steve Maynard: 253-597-8647
steve.maynard@mail.tribnet.com

(Published 12:01AM, November 3rd, 2003)

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